High-tension cable



July 3, 1928. 1,675,918 R. W. ATKINSON ET AL HIGH TENSION CABLE Filed May 9, 1923 INVENTOR w/nvzssss /?KJ w, (if/mm, M &? yaw-W W 2 Patented July 3, 1928.

UNITED STATES- I l,675,918 PATENT OFFICE.

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HIGH-TENSION cunm.

Application filed lay 9, 192a. Serial no. seam.

Our invention relates to the construction of multiple-conductor electrical cables, and particularly of those intended for high-voltage work, and the object in view is preven- 5 tion of injury in manufacture, installation and in service. 4

Therinvention is illustrated in the accompanying drawings. Figs. I and II are views 1n transverse section of cables embodying our invention, but differing one from another in detail, and so serving to indicate that in application the invention admits of some latitude in detail; Fig. III is a view in perspective of a cable embodying the-invention, it happens to be the cable shown in Fig. II. The sheath is, for illustrative purposes broken away.

The usual parts of a multiple conductor high-tension cable are the conductors themselves, with their individual envelopes of insulation, the fillers between the assembled individually insulated conductors, the belt A insulation surrounding'conductors and fillers when assembled, and the sheath commonly of lead, encasing the whole. In the drawings the conductors are indicated at 1,

their envelopes of insulation at 2, the fillers at 3, the belt insulation at 4, and the lead sheath at 5. The arrangement shown in the drawings of the conductors within the cable is the usual one; the conductors are eccentricall assembled, and in the three-conductor ca le shown, the centers of the conductors stand in the relative positions of the apices of an equilateral triangle, and the center of the triangle is in the axis of the cable.

The belt insulation is a part of the structure which may or may not be present, and its thickness when present may vary widely,-this whole matter being dependent on and conditioned by the use to which the cable is to be put, the service which it is to render. Ordinarily belt insulation is present, but its elimination, or even diminution in thickness, brings to light a mechanical purpose, quite distinct from its electrical purpose, which the belt insulation ordinarily serves, without attracting attention; the purpose, namely, of mechanically holding the assembled cable parts in proper relative positions. This need for holding the parts assembled is most obvious when it comes to applying the lead sheath; and then, if there be no belt insulation, it is common to bind the assembled parts together by a winding of suitable material, usually cloth-tape, adequate to holdthe parts in place till the lead sheath is applied. When the sheath has then been applied, it becomes the effective holding means.

The necessity for mechanical security sometimes dictates a heavier belt insulation than for electrical purposes is needed. We have perceived that greater mechanical security is desirable, not only when applying the lead sheath, but even after the lead sheath has been applied. The lead sheath itself is inadequate to hold with certainty the cable parts when installation 'is being made and when the vicissitudes of service are encountered. When installation is being made, a cable ordinarily must be manipulated and bent more or less, and strains incident to bending tend to distort the cable from its proper truly cylindrical form, and to open up spaces within it. Such spaces, when they develop, are places of weakness, for in high-voltage service corona discharge occurs within them, and sooner or later a breakdown of insulation ensues,a shortcircuit path develops.

Such spaces may also be produced or accentuated by anot er condition. When the conductors of a cable are carrying their normal current, there is a repulsive force set up between them, that is a force tending to drive them apart, a force which is exerted on the binding envelope, whatever it may be, tending to disrupt it. Normally this disruptive strain is of small magnitude, and of no practical consequence. But the force is one whose magnitude varies as the square of the current flowing through, and cables in service often have to carry abnormally for brief periods of time currents which are tens of times greater than normal. So that under service conditions the binding element must be strong to resist abnormal disruptive strains, hundreds of times greater than normal. And in the usual cable construction the consequence of disruptive strains of the nature indicated may be very serious, opening spaces, distorting the structure, injuring the sheath, and eventually bringlng about complete destruction and failure. This disruptive strain becomes a matter of articular importance in the case of a multip e-conduc- 'permanent wrapping whose purpose is purely mechanical, adequate not merely to hold the parts in place while the lead sheath is being applied, but adequate additionally to resist and overcome strains which in the operation of making installation and in ser ice tend to cause displacement of parts. Th s permanent wrapping which constltutes our invention is in preferred form a ribbon of steel. Such a ribbon, properly applied is in the drawings indicated at 6. n the cable of Fig. I (no belt installation being present), it overlies and is applied directly upon the individually insulated conductors and the fillers, when assembledjin the cable of Figs. II and III it overlies the belt insulation 4. In both instances the lead sheath 5 overlies it.

The engineer will immediately perceive that with such a wrapping of great tensile strength added to the structure, no thought need be given'to any mechanical function served by belt insulation; the belt insulation may b formed and proportioned, or even wholly omitted, as electrical requirements alone may dictate, and no thought need be given as to how the parts are to be held in place while the sheath is being applied in the lead press.

When the cable parts are so secured by the wrapping of our invention, there is no need to resort to the expedient often employed, of building up belt insulation to a thickness greater than is electrically required. And there is great advantage in this, for wrappings of belt insulation are relatively bulky, while such a steel ribbon as we employ adds practically nothing to the size of the structure; and, accordingly, the practice of our invention effects (other things being equal) a very substantial, and therefor very considerable, saving in the size and cost of the cable. 7

A steel strip is, mechanically considered, so vastly stronger than the wrapping used in building belt insulation, and is so vastly stronger than a lead sheath, to resist strains of the nature spoken of above, that the dangers incident to installation and service are fully guarded against, and both insulation and sheath may be designed with their other purposes in mind, wholly freed from this matter of overcoming internal strains.

The steel strip is in no respect disadvantageous, because of conductive or magnetic properties.

It is a practical necessity that the pitch at which the tape '6 or its equivalent, is wrapped upon. the cable body shall be suflicient to attain in appreciable degree the end in view. The effect varies as the square of the sine of the angle at which the tape I is so slight as to be of no practical value in attaining our ends. We therefore in certain ensuing claims qualify our invention by defining it as limited to a wrap laid upon the cable body at an angle to the length of the cable body exceeding 15.

We have described our invention as embodied in a wrapping in the form of a steel ribbon. Other material of high tensile strength may of course be employed, copper, for instance; but no other material capable of being brought to the desired form, is at once so strong and so cheap as steel. The

wrapping need not be specifically of ribbon form; it may, for instance be a wire, or it may be in the form of a woven tape. The successive turns of the wrapper need not be, and, with economy of material in mind, should not be in contiguity, turn upon turn. Fig. III of the drawing indicates this.

We claim as our invention:

1. A multiple-conductor electrical cable of maximum carrying capacity and minimum diameter, including in its structure a cable body formed of a plurality of insulated conductors eccentrically assembled within a lead sheath, means for preventing separation of the conductors, such means consisting of a band of metal surrounding the body of assembled conductors and extending at an angle to the length of the cable body exceeding fifteen degrees, the sheath enveloping such banded cable body.

2. A multiple-conductor cable of maximum carrying capacity and minimum diameter, including in its structure a cable body formed of a plurality'of insulated conductors eccentrically assembled, and including also belt insulation enveloping the assembled conductors, means for preventing separation of the conductors consisting of a band of metal laid upon and surrounding the beltinsulated cable body, the said band extending upon the cable body at an angle to the length of the cable body exceeding fifteen degrees, and a sheath enveloping the belted and banded cable body.

In testimony whereof we have hereunto set our hands. 

